Surfactant is generally defined by containing both hydrophilic group (head) and hydrophobic group (tail) which has an orientation to form aggregates or micelles in aqueous solutions. The amphiphilic properties of surfactant significantly lower the surface tension or interfacial tension between two liquids or between a liquid and a solid material. Owing to their functional properties resulted by chemical and physical actions, such as wetting, anti-sticking, emulsifying/demulsifying, foaming/defoaming, solubilizing, dispersing, cleaning, antiseptic, and antistatic, surfactants are applied extensively and practically, and thus become a fine chemical engineering agent with diversified usability.
Surfactants have been developed for more than a half century. Surfactants were initially used for detergents, and they are applied in all fields of fine chemical engineering nowadays. Surfactants are developed rapidly in cosmetic industry, and become the most important component of cosmetic ingredients, which play crucial roles in cosmetics including the formulation, the manufacture, the carriage, and the usage. In order to obtain ideal performance of cosmetics consistently to finally satisfy the sensory demands of customers, people have to control the parameters of rheological property critically.
However, most of current surfactants irritate skins to a varying extent, and even cause acute symptoms of skin allergies. The dissolution, infiltration and reactivity of surfactants can induce mucosal irritation; and it is generally accepted that the direct cause is the infiltrated residues of the surfactants. A. Mehling and colleagues have studied the ocular and dermal (skin) irritation potential of surfactants by using various methods such as the red blood cell test, the hen's egg test-chorioallantoic membrane (HET-CAM), the Skinethic ocular tissue model, and 24 h epicutaneous patch test (ECT). The results indicate that coco-glucoside, lauryl D-glucoside, lauryl glucose carboxylate, sodium dodecylpoly(oxyethylene) sulfate, sodium lauryl sulfate, ammonium lauryl sulfate, and other tested irritant surfactants can potentially cause mild, moderate, or even severe irritation.
Chlorinated organic surfactants and phenolic surfactants may cause defects in fetus and tumor formation in animal models. For example, sodium dodecyl benzene sulfonate can be absorbed dermally (through skin) to cause chronic symptoms such as liver lesion and splenic atrophy, and those surfactants are teratogenic and carcinogenic.
Low molecular polyethylene oxide belongs to nonionic surfactant, which is considered to be mutagenic and thus brings up safety concerns. For example, when conducting addition reaction with ethylene oxide (EO), excessive ethylene oxide results in unreacted ethylene oxide, low molecular polyethylene and 1,4-dioxane cyclized by diethyl dioxide.
In addition to the potential hazard through skin contact, surfactants can be environmentally hazardous because the surfactants are discharged down the drain with the dirty wash water. The bio-safety concerns of various surfactants widely used today including nonylphenol ethoxylate (NPE), diethanolamine (DEA) and linear alkylbenzene/linear alkylbenzene sulfonate (LAB/LAS) are existing unsolved problems. According to the current European standard, environmentally-safe surfactants are required to have 90% of average biodegradability, and 80% of initial biodegradability.
In view of above, the awakened public environmental consciousness and the heightened bio-safety criteria make cosmetic industry to choose the surfactant cautiously. It is observed that the issues of regulation and environmental protection increasingly limit the development of surfactants, which impacts significantly on the cosmetic industry supplying products applied to human body.
Therefore, people are seeking surfactants of low dermal irritancy, non-toxicity (including teratogenicity, mutagenicity and carcinogenicity), and biodegradability.